1. Field of the Invention
This invention relates to the preparation of reaction injection molded polyurethane elastomer. It more particularly relates to the preparation of reaction injection molded polyurethane elastomers which have improved thermal dimensionally stable characteristics.
2. Description of the Prior Art
The automotive industry is faced with legislative mandates which require improved fuel economy standards. In order to achieve these higher fuel economy goals, the automotive industry has downsized large vehicles. Furthermore, the automotive industry has investigated the use of lower weight materials. Among the types of materials which may be employed are those produced by reaction injection molded polyurethanes. In order for these products to meet the demanding requirements for their application, they must be sufficiently rigid to be self supporting, have thermal dimensional stability to allow for normal processing operations at elevated temperatures, have low coefficients of thermal expansion, have a class A surface and good paintability, and enjoy good impact characteristics at low temperatures. Low coefficients of thermal expansion may also be improved by the addition of milled glass fibers to the polyurethane matrix. Further, in order to enjoy the required thermal dimensional stability to allow the processor to, for example, paint at elevated temperatures, the urethane products must pass a heat sag test in the neighborhood of 165.degree. C. in order to be practical for use in the painting applications as practiced by the automotive industry.
The reaction injection molded polyurethanes of the instant invention are generally prepared by reacting a mixture of polyoxyalkylene polyether polyol with various polyisocyanates and incorporating therein chain extenders such as ethylene glycol and/or butanediol while maintaining a low free water content prior to annealing of the elastomer.
U.S. Pat. No. 3,892,691 teaches the preparation of polyurethane products employing quasi prepolymers of diphenylmethanediisocyanate and dipropylene glycols together with a polypropylene ether triol such as is prepared by the reaction of ethylene and propylene oxide with trimethylolpropane or glycerol and the use of the chain extender 1,4-butanediol.
U.S. Pat. No. 4,243,760 teaches the preparation of reaction injection molded polyurethane products by employing chain extending agents such as ethylene glycol, propylene glycol and 1,4-butanediol.
U.S. Pat. No. 4,102,833 also teaches the preparation of reaction injection molded urethanes by employing long chain polyols together with a short chain diol or triol such as ethylene glycol or glycerol.
None of the prior art, however, recognizes the necessity for preparing and maintaining a very low free-water content in the resultant elastomer prior to annealing of the elastomer at elevated temperatures in order to obtain thermal dimensional stability.